4 POTABLE WATERS
(4) Soap solution : 150 grams of ordinary lead soap and 40 grams of
pure dry potassium carbonate are triturated in a mortar until homogeneous
and then exhausted with 96% alcohol. The filtered alcoholic solution is
distilled to eliminate the solvent and of the residual soap, dried on a steam-
bath, 20 parts are dissolved in 1000 parts of 56% alcohol. The soap solu-
tion thus obtained is then titrated as follows. One hundred c.c. of the
barium chloride solution are introduced into the bottle, the soap solution
being then run in gradually from the burette and the bottle closed and
shaken from time to time until a froth 5-6 mm. high, persistent for 5 minutes,
is formed. The soap solution is then either diluted with 56% alcohol or
concentrated by addition of soap until exactly 45 c.c. correspond with 100
c.c. of the barium chloride solution.
The hardness of a water is determined similarly, 100 c.c. of the water
being taken, or, with very hard waters which produce clots, 50, 20 or 10
c.c., or, in general, such volume as requires from 20 to 45 c.c. of the soap
solution ; this volume is made up to 100 c.c. with distilled water before
titration.1 The result obtained is then corrected for the dilution.
If the water is very rich in magnesium salts, it is well to wait a few
minutes after each addition of soap solution and before shaking in order
that all the magnesium may combine with the soap.
From the number of degrees of soap solution necessary to produce a
persistent froth with 100 c.c. of the water the hardness in German degrees is
calculated by means of Table I. One German degree represents I gram
Clark's Hardness Table., compiled by Fuisst and Knauss,
Soap Solution used in c.c.
Corresponding degrees of hardness.
Number < if degrees corresponding with i c.c. of soap solution,
Soup Solution used in c.c.
C'orrespoud-ing degrees of hardness.
Number of degrees corresponding with T e,.e, of soap solution.
3 3 '3
^To judge of the quantity of water to be taken, a preliminary test is made by
shaking in a test tube 20 c.c. of the water with 6 c.c. of the soap solution. If the liquid
becomes opalescent, 100 c.c. of the water may be taken ; if it becomes very turbid,
50 c.c., or, if it gives a precipitate, 20 or 10 c.c. according to the amount of the pre-
cipitate.is requires :